Portable life support systems are used in a variety of situations in which the ambient environment around the user cannot be breathed either because of the lack of oxygen in usable form or because of the presence of substances which would have toxic effects if inhaled. These uses include extravehicular activity in space, scuba diving, including deep off-shore diving work, use in contaminated atmospheres, use at high altitudes and the like.
The two fundamental architectures in the design of portable life support apparatus are open circuit and closed circuit systems. Open circuit systems, typified by the underwater diving system popularized by Jacques Cousteau, are the simplest, consisting of a compressed gas supply and a demand regulator from which the user breathes. The exhaust gas is ported overboard with each breath, hence the name "open" circuit. These systems are bulky and inefficient in that the oxygen not absorbed during each breath is expelled and wasted. Additionally failure of any component results in failure of the system.
Closed circuit systems, also known as rebreathers, make nearly total use of the oxygen content of the supply gas by removing the carbon dioxide generated by the user, and adding makeup oxygen or oxygen containing gas to the system when the internal volume drops below a set minimum level, or when the oxygen partial pressure drops below some pre-established setpoint.
These closed circuit breathing systems generally consist of a mouthpiece from which the user breathes and which is connected by means of two flexible impermeable hoses, one to remove the exhaled gas and the other to return the processed gas, to a means for removing the carbon dioxide from the breathing gas, replenishing metabolized oxygen, and providing for makeup gas volume with a breathable gas to maintain system volume during descent as the gases within the breathing circuit are compressed. Such devices are usually provided with a series of checkvalves located near the mouthpiece such that gas flow within the breathing circuit is always maintained in a single direction. Oxygen addition to the system may be made by oxygen generators, such as the type disclosed in U.S. Pat. No. 2,710,003, to Hamilton et al., or the addition of oxygen or an oxygen containing gas either through a constant mass flow orifice or by means of a manually operated or a sensor-controlled electronic valve.
Gas addition closed circuit systems may be one of two types, a pure oxygen version, which is limited to operating environments where the partial pressure of oxygen is less than two atmospheres, and a mixed gas version, normally used for underwater work at great depths. From a control standpoint, oxygen rebreathers are quite simple and require no active control. Mixed gas rebreathers, on the other hand, are considerably more complex. These were first pioneered in the late 1960's in an effort to solve the problems of narcosis at depths and to eliminate the oxygen toxicity problems which limit the safe diving depth of pure oxygen rebreathers.
When breathing in a closed circuit system, the exhaled breathing gas is held in a closed container, such as a breathing bag or a counterlung. Work is done when the gas is exhaled into, or inhaled from, the counterlung since surrounding environment is displaced as the counterlung is expanded. It has now been discovered that the work of breathing is dependent upon the user orientation angle and is directly related to static lung loading, which is the vertical distance, in centimeters of water, from the user's or "diver's" suprasternal notch, and the center of gravity of the inflated counterlung. Further, lung physiology prefers a slight positive pressure during inhalation, such as a static lung loading of between 0 to +10 centimeters of water. The present invention is the first to appreciate that known rebreathers with back-mounted counterlungs have negative static lung loadings and thus difficult inhalation characteristics while those that are chest-mounted have positive static lung loadings well in excess of +10 centimeters of water, and thus have hard exhalation characteristics. Furthermore, it has also been discovered that these known counterlungs are very sensitive to the user orientation angle due to the location of the center of gravity of these counterlungs.
In the prior art manual bypass valves, which permit the user to manually add either oxygen or an oxygen containing gas to the breathing circuit in the event of failure of the automatic valves, if present, have been placed on the body of the rebreather. For the case of a back-mounted rebreather, such as that shown in U.S. Pat. No. 3,710,553, these valves require an awkward reverse reach in order to operate them.
The major deficiencies and problems existing with these known systems include a lack of redundancy or safety, limited duration or range, excess weight, high breathing resistance, and difficult manual operation.
A major leak anywhere in the breathing circuit of existing rebreathers leads to a subsequent flooding of the carbon dioxide removal system and therefore failure of the breathing apparatus. For operations conducted in locations where an immediate abort to a safe environment is impossible, such a failure could result in the death of the diver.